The present invention relates generally to the art of self-contained heating units and in particular to heating units which employ a chemical reaction to produce heat. Several such heating units are disclosed in the prior art for such diverse heating applications as warming foods or drinks, heating shaving water, heating hair curlers, providing concentrated heat for small soldering jobs, etc. A description of several such prior art units can be found in a copending U.S. patent application, Ser. No. 545,206, filed Jan. 29, 1975 now abandoned by John H. Trumble, Dr. Thomas C. Ehlert and Akos Szekely, entitled "Heating Device," which application is assigned to the assignee of the present invention and is expressly incorporated herein by this reference. As disclosed in such copending application, the prior art systems suffered from one or more disadvantages including toxicity of the chemicals, lack of the ability to control the reaction rate and heat output, ignition problems, excessive gas pressures in the heating cartridges, etc.
The copending application also discloses that several such prior art devices have employed the "Thermite" reaction for producing heat. This reaction can be represented by either of the following formulas: EQU 2Al + Fe.sub.2 O.sub.3 .fwdarw. 2 Fe + Al.sub.2 O.sub.3 + heat EQU 8Al + 3Fe.sub.3 O.sub.4 .fwdarw. 9 Fe + 4 Al.sub.2 O.sub.3 + heat
This reaction has received much attention because the iron oxide supplies the reaction supporting oxygen for the system, so the reaction needs no outside gas source and produces no gaseous products. However, as disclosed in the copending application, the prior art thermite heaters are impractical in that the reaction rate in such systems is so fast that the available heat cannot be transferred from the heat source to the substance to be heated without boiling, burning or scorching it.
The copending application relates to an invention which comprises thermally and/or chemically controlling the reaction burn rate of such chemical heater cartridges, and is based on the discovery that the burn time of such cartridges is dependent not only on the chemical mixture, but on the temperature and heat conductivity characteristics of the cartridge shell, the sleeve surrounding the cartridge and the substance being heated. Thermal control is accomplished by dividing the chemical mixture into a plurality of compartments and causing a series reaction through the compartments to extend the reaction time or by providing a plug of inert material at the core of the reactive chemicals. Chemical control is accomplished by adjusting the reaction stoichiometry or by adding diluents to the chemical mixture. Either or both of these techniques has been found to be effective in maximizing heat transfer, especially to liquids and semi-solid food products.
While the invention disclosed in the aforementioned application permits preparation of self-contained heating units which are useful and which possess numerous advantages over prior art heaters, ways of improving the efficiency of heat transfer have been investigated. When employing a metallic cylindrical sleeve snugly surrounding a cartridge containing alumino-thermic reactive chemicals, the sleeve rapidly transfers the heat generated within the cartridge to the external surface of the sleeve, and it has been found that the external sleeve temperature exceeds the scorching temperature of some liquids, especially milk or milk-based food products. This is the case even when using sleeves having external fins such as the sleeve disclosed in the drawings of the aforesaid application. Moreover, the heat is not evenly distributed over the entire surface of the sleeve because, as the chemical reaction progresses through the length of the heating cartridge, temperature measurements have established that the sleeve is hottest at the area thereof adjoining that portion of chemicals then reacting in the cartridge. This hot spot moves down the sleeve until the reaction is completed. It has also been determined that the problem is most pronounced at the middle portion of the sleeve. A sleeve design which overcomes these problems would be a significant advance in this technology.